1. Field of the Invention
The present invention relates to an optical variable attenuator and an optical module which are used in optical communications and the like.
2. Related Background Art
Known as an example of conventional optical variable attenuators is one disclosed in the IEICE Technical Report, PS2001-31. The optical variable attenuator described in the publication is one in which two optical fibers oppose each other, whereas a shutter is put into/out of the space therebetween, so as to control the optical attenuation amount.
The above-mentioned prior art uses two shutters in order to reduce the polarization-dependent loss (wavelength dependence of attenuation amount) caused by the diffraction of optical beams at edge parts of a shutter. In this case, however, actuators for separately driving the two shutters are necessary, whereby the structure of optical variable attenuator becomes complicated/bulky.
It is an object of the present invention to provide an optical variable attenuator and an optical module which, while in a simple structure, can reliably reduce the polarization-dependent loss.
The optical variable attenuator of the present invention comprises a base member having an input optical line and an output optical line; a movable mirror for reflecting light passing through the input optical line toward the output optical line; and driving means for moving the movable mirror.
When the movable mirror is located at a predetermined position in such an optical variable attenuator, the light from the input optical line is totally reflected by the movable mirror so as to be guided to the output optical line, whereby a state with the lowest optical attenuation amount is attained. When the movable mirror is moved by the driving means from this state, only a part of the light from the input optical line is reflected by the movable mirror, whereby the optical attenuation amount increases. When the movable mirror reaches a position completely displaced from the input optical line and output optical line, the movable mirror reflects no light at all, so that no reflected light is guided to the output optical line, whereby a state with infinite attenuation is attained. Since the optical variable attenuator of the present invention utilizes the reflection of light effected by a movable mirror, it is free from influences of diffraction of light at edge parts of the movable mirror. Also, a single movable mirror is enough, unlike cases where a simple shutter is used as a member for changing the optical attenuation amount. Therefore, a simple structure can reliably reduce the polarization-dependent loss caused by light diffraction effects. In this case, the polarization-dependent loss at an optical attenuation amount of 10 dB can be made smaller than 0.2 dB.
Preferably, the movable mirror is formed by a microelectromechanical technology. This can reduce the size of the movable mirror, whereby the optical variable attenuator can be made smaller.
Preferably, the input optical line and output optical line are formed by an optical waveguide, the base member is provided with a groove connecting with the input optical line and the output optical line; and the movable mirror is provided so as to be movable in a state inserted in the groove. When the input and output optical lines are formed by an optical waveguide as such, the base member having the input and output optical lines can easily be made by using a semiconductor manufacturing technology, whereby the cost can be cut down.
Preferably, in this case, the input optical line and output optical line form a virtual intersection angle of at least 9 degrees therebetween. Here, the virtual intersection angle refers to an angle at which the input and output optical lines intersect if they are extended straight as they are. When such a virtual intersection angle is at least 9 degrees, the light returning to the input optical line after being reflected by the movable mirror is reduced, whereby the optical loss caused by return light can be suppressed to a low level.
Preferably, the groove is filled with a matching oil having a refractive index on a par with that of the optical waveguide. This can reduce the optical insertion loss between the input and output optical lines and the movable mirror.
Preferably, the movable mirror is attached to a cantilever supported on the base member, whereas the driving means has an electrode disposed on the base member and means for generating an electrostatic force between the cantilever and the electrode. Such a configuration driving the movable mirror by utilizing an electrostatic force makes it unnecessary for currents to flow, whereby the power consumption can be lowered.
Preferably, in this case, the cantilever is provided with a plurality of first teeth, whereas the electrode is provided with a plurality of second teeth alternately inserted between the first teeth. This increases the surface areas of the cantilever and electrode, whereby a large electrostatic force can be generated between the cantilever and electrode with a low voltage applied thereto. Also, since the cantilever and the electrode are disposed closer to each other, the linearity of amount of movement of the movable mirror with respect to the applied voltage, i.e., the linearity of optical attenuation amount with respect to the applied voltage, improves. Therefore, it becomes easier to control the optical attenuation amount.
Preferably, the movable mirror, cantilever, and electrode are formed from Si having a conductivity, whereas the movable mirror has a surface coated with one of Au, Ag, and Al. When the movable mirror, cantilever, and electrode are formed from Si having a conductivity as such, these structures can be made inexpensively. When a surface of the movable mirror is coated with one of Au, Ag, and Al, a high-performance movable mirror with a favorable reflectance can be obtained.
The optical module of the present invention comprises the optical variable attenuator mentioned above. When provided with the optical variable attenuator adjusting the optical attenuation amount by utilizing the reflection of light caused by the movable mirror as such, the polarization-dependent loss caused by light diffraction effects can reliably be reduced by a simple structure as mentioned above.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.